In previous studies, we have examined the function and regulation of the soluble form of the IL-4R. Using a cDNA probe that is specific for the soluble IL-4 receptor, we have shown that resting bone marrow-derived macrophages or Ml cells do not express detectable levels of soluble IL-4R unless stimulated with either IL-6 or IFN-gamma. When stimulated, they undergo a rapid upregulation of soluble IL-4R expression at both the level of the mRNA and at the level of the protein. Studies are currently underway to determine the mechanism of this induction of soluble IL-4R, and to determine the physiologic role that soluble IL-4R may play in the immune response.